The easiest way to tell an arthropod from any other animal is to see if they have:

1) A segmented body.

This means that they will have a body made up
of more than one part. Spiders have two segments and flies have three segments.

centipede

2) Many jointed legs or limbs.

Spiders have 8 legs, millipedes can
have... Hundreds!

CrabPhoto Source: Corel Web Gallery

3) An exoskeleton.

This is an external skeleton. Like armor, it protects the arthropods body. When arthropods are born the exoskeleton is soft but hardens quickly and it can be shed as the creature grows.
Arthropods are invertebrates; which means that they do not have a backbone.

Photo Source: Corel Web Gallery

4) Cold blooded

Arthropods are cold blooded -- which means,
their body temperature depends on the temperature of the environment
surrounding them.

Photo Source: Corel Web Gallery

Arthropods are some of the most interesting animals in
the world!

They fly, they creep, and they crawl.
They live on land, in ponds and in the ocean. From ants to bumblebees, crabs
to crayfish, spiders to
centipedes -- which are your favorites!?

Scientific stuff: Arthropods include eleven animal
classes

Subphylum Chelicerata

Class Merostomata (horseshoe crabs, eurypterids)

Class Pycnogonida (sea spiders)

Class Arachnida (spiders, ticks, mites)

Subphylum Crustacea

Class Remipedia

Class Cephalocarida

Class Branchiopoda (fairy shrimp, water fleas)

Class Maxillopoda (ostracods, copepods, barnacles)

Class Malacostraca (isopods, amphipods, krill, crabs,
shrimp)

Subphylum Uniramia

Class Chilopoda (centipedes)

Class Diplopoda (millipedes)

Class Insecta (all of the insects including ants, bees,
beetles and butterflies)